Light emitting devices comprise at least one light emitting diode (LED) which generally is arranged with an optical element. The optical element generally comprises a wavelength converting layer comprising a phosphor and/or a layer that redirects the emitted light. Thereby, the primary light emitted from the LED can be influenced in various ways.
A top-emitting LED refers to a light emitting diode having an optical element, wherein the light generated by the LED is illuminated at the top surface of the optical element. For optical elements in top-emitting LEDs, properties such a high transparency and optimal wavelength conversion are desired. This is preferably achieved by using an optical element comprising a wavelength converting layer, in which a light beam having a first wavelength is transformed by phosphor to a second wavelength and then transmitted through the layer.
A side-emitting LED refers to a light emitting diode having an optical element, wherein the light generated by the LED is illuminated at the side surfaces of the optical element. For optical elements in side-emitting LEDs, properties such a high transparency and optimal wavelength conversion generally are desired at the lower part of the optical element, while high reflectivity of the upper outer part of the optical element generally is desired. This is preferably achieved by using an optical element comprising a first wavelength converting layer, in which the light is transformed by phosphor and then reflected by a second layer.
US 2007/0284600 A1 describes a side-emitting LED comprising a wavelength converting layer and a reflector. The wavelength converting layer may be sintered phosphor particles or phosphor particles in a transparent or translucent binder, which may be organic or inorganic. The reflector can be a specular or diffusing reflector which causes the light to be reflected at many angles and ultimately at less than a critical angle. It is described that the specular reflector can be formed from organic or inorganic layers, for example aluminum or another reflective metal. Further, it is described that a diffusing reflector may be formed of a metal deposited on a roughened surface or a diffusing material, such as a suitable white paint.
There are several issues that have to be considered when optical elements comprise both a wavelength converting layer and a scattering layer. For example, it is desired to obtain good physical and optical contact; and thermal stability and to avoid chemical degradation.
Good physical contact between the wavelength converting layer and the scattering layer is of critical importance, in order to avoid mechanical failure, such as delamination, of the optical element. Currently, an additional adhesive layer is frequently used in order to achieve sufficient adhesion of the layers in the optical element. The optical element is exposed to temperature variations during use, which may result in heat-induced stresses caused by the use of materials with different thermal expansion properties in the wavelength converting layer and the scattering layer.
Good optical contact in-between the wavelength converting layer and the scattering layer is of importance since even a very fine gap between the two layers can cause light leakage at the sides in undesired angles. This may result in undesired deviations from the regular light intensity distribution.
In addition, chemical interactions between the wavelength converting layer and the scattering layer may result in degraded function of the optical element, such as destroyed luminescence or a change of the emission color.
Therefore, there is a need in the art for providing an optical element comprising a wavelength converting layer and a scattering layer for a light emitting device, and a manufacturing method thereof, wherein these disadvantages are overcome.